Paper lab-on-a-chip makes disease tests affordable

Simply putting pen to paper on a sunny day offers a way to make lab devices to help test for disease.

Devices for testing water quality and identifying pathogens can now be made using the right paper, ink and sunlight. The method could help developing countries access the latest lab techniques.

Microfluidic chips are blocks of glass or plastic containing labyrinths of tiny channels no more than a few hundred micrometres in width.

These channels can manipulate fluids with great precision, thanks to capillary forces that draw liquid through the chip and a lack of turbulence at small scales. Such chips offer a way to shrink down and streamline lab procedures involving several steps, such as testing for disease bacteria DNA. Several solutions can be loaded into one chip, and the results read out not long after.

However, making these delicate devices is complicated and expensive, so they do not reach remote areas of the developing world where they offer greatest benefit.

Usually, a copper mould of each design is needed to cast the chips out of plastic. Now, though, researchers at Harvard have developed a simple way to make the devices out of paper instead.

Masked marvel

George Whitesides and colleagues realised that the pores naturally present in paper can carry liquids in a similar way to standard chips.

All they needed was a way to make impermeable "walls" to confine the fluids into narrow channels. They achieved this using a polymer liquid that sets hard when exposed to UV light, called photoresist. It is used in industry to etch designs onto silicon chips.

To make the paper chips, the researchers soak a piece of normal office paper in photoresist. They then cover one side with a transparency and draw the desired pattern of channels on top with a pen.

When the paper is exposed to sunlight, the photoresist sets hard, apart from the area of the paper that is masked beneath the inked-on pattern. The transparency is then peeled off, and the paper quickly washed to remove any still-liquid photoresist.

The final product, once dried, is a sheet of impermeable paper with a tiny network of permeable channels embedded within it - in other words, a microfluidic device.

Drawing onto the transparency by hand is, of course, not very precise. But Whitesides says designs could simply be printed onto the transparency using an inkjet printer.

Thinking ink

"Paper-based microfluidic devices probably won't be sensitive enough to detect every disease, but they are sensitive enough for many useful applications," says Andres Martinez, one of Whitesides' colleagues.

Aaron Wheeler, a microfluidics expert at the University of Toronto in Ontario, Canada, is impressed with the idea. The Whitesides group has helped popularise microfluidics over the last decade, he says.

Wheeler is also interested in making microfluidic devices more accessible. And he, too, is using ink, but his team is attempting to find a cheap and simple way to create the traditional copper moulds.

In his latest work (Lab on a Chip, DOI: 10.1039/b804050h), a pattern is inked directly onto the sheet of copper before a chemical is used to etch away a thin layer from exposed areas. Copper beneath the ink remains intact, forming an embossed pattern that is used to cast microfluidic devices from plastic.

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A microfluidic device in paper: the "Africa" pattern is placed over photoresist-soaked paper. Where exposed to sunlight, the resist becomes impermeable, leaving a network of channels (Image: Lab on a Chip)